1. Field of the Invention
The present invention relates to a piezoelectric resonator, using a piezoelectric thin film, usable for wireless communication apparatuses represented by, for example, mobile phones and wireless LAN apparatuses, and a method for producing such a piezoelectric resonator.
2. Description of the Background Art
Components built in mobile communication apparatuses or the like are demanded to be more compact and more lightweight while having high performance. For example, filters or duplexers used in mobile phones for selecting RF signals are demanded to be compact and small in insertion loss. One known filter for fulfilling such demands is a filter including a piezoelectric resonator using a piezoelectric thin film. See, for example, Japanese Laid-Open Patent Publication No. 2002-374144.
FIG. 8A is a plan view schematically showing a structure of a conventional piezoelectric resonator. FIG. 8B is a cross-sectional view of the conventional piezoelectric resonator shown in FIG. 8A taken along line Y-Y in FIG. 8A.
The conventional piezoelectric resonator includes a substrate 705, a lower electrode 702 provided on the substrate 705, a piezoelectric layer 701 provided on the lower electrode 702, and an upper electrode 703 provided on the piezoelectric layer 701. An area in which the lower electrode 702, the piezoelectric layer 701 and the upper electrode 703 overlap in a vertical projection direction acts as a vibration section 704. In the substrate 705, a cavity 706 for guaranteeing the vibration of the vibration section 704 is formed. The vibration section 704 is positioned on the substrate 705 so as to cover the cavity 706. Owing to such a structure, a part of the lower electrode 702 and/or the piezoelectric layer 701 which is not included in the vibration section 704 acts as a support section for supporting the piezoelectric resonator on the substrate 705. In an actual product, line electrodes 708 are provided for respectively connecting the lower electrode 702 and the upper electrode 703 to input/output electrodes 707.
The conventional piezoelectric resonator having such a structure is produced as follows.
First, the lower electrode 702, the piezoelectric layer 701, and the upper electrode 703 are sequentially formed on the substrate 705 by vapor deposition or sputtering. Next, the upper electrode 703, the piezoelectric layer 701 and the lower electrode 702 are patterned into a desired shape by photolithography. In the case where the input/output electrodes 707 and the line electrodes 708 are formed of the same material as that of the lower electrode 702 and the upper electrode 703, the input/output electrodes 707 and the line electrodes 708 are formed at the time of the patterning. Finally, the cavity 706 is formed by dry etching or the like performed from a surface of the substrate 705 on which the above-mentioned elements are not provided.
The piezoelectric resonator having such a structure operates as follows. An electric field is applied between the lower electrode 702 and the upper electrode 703 to mechanically distort the piezoelectric layer 701. The distortion is converted into an electric signal. Thus, the piezoelectric resonator acts as a resonator. There are a great number of vibration modes by the piezoelectric layer 701, but a thickness longitudinal mode having a high resonance frequency is used for wireless communication apparatuses represented by, for example, mobile phones and wireless LAN apparatuses.
As is well known, when a piezoelectric resonator is in a free vibration state in which the vibration section is floating in the air, the Q value which represents the resonance sharpness, and Δf which is the difference between the resonance frequency and the anti-resonance frequency, are optimum. However, it is impossible to produce a vibration section which is physically floating in the air, and a support section is indispensable.
In the above-described conventional piezoelectric resonator, a part of the lower electrode 702 is in contact with the substrate 705 acting as a support section. Therefore, there is a problem that the vibration of the vibration section 704 leaks to the substrate 705 and thus the frequency characteristic is deteriorated. Even in a structure in which a part of the lower electrode 702 is not in contact with the substrate 705, the line electrode 708 is in contact with the substrate 705. Therefore, there is also the problem that the vibration of the vibration section 704 leaks to the substrate 705 and thus the frequency characteristic is deteriorated.